In recent years, image pickup apparatuses such as digital still cameras and digital video cameras which capture images using image pickup elements and store the captured images as digital data have been widely used. CCD (Charge Coupled Device) image sensors have been commonly used as image pickup elements included in such image pickup apparatuses. However, as the number of pixels included in the image pickup element is further increased, CMOS (Complementary Metal Oxide Semiconductor) image sensors have attracted more attention. When compared with CCD image sensors, CMOS image sensors are characterized by random access of pixel signals, higher reading speed, high sensibility, and low power consumption.
Regardless of whether a CMOS sensor or a CCD sensor is used in an image pickup apparatus, when a mechanical shutter is used to control an exposure time, variation of the exposure time occurs due to a variation error of positional accuracy which occurs at a time of assembly or a variation error of accuracy of operations of shutter blades. In particular, when a high-speed shutter is used, a percentage of an error in the exposure time is large.
On the other hand, most image pickup elements have electronic shutter functions. In such an electronic shutter function, pixels of an image pickup element are reset before exposure is started, and signals of the pixels of the image pickup element are read before the exposure is terminated. In this manner, since the start of the exposure and the end of the exposure are controlled only using a function of the image pickup element, reliable control of the exposure time of a low-speed shutter and a high-speed shutter can be realized.
However, an electronic shutter function of the CMOS image sensors is different from that of the CCD image sensors in that a number of pixels arranged in a matrix are successively scanned for each pixel row so that signals are read. Accordingly, in a CMOS sensor, the exposure time is varied according to rows (refer to FIG. 9 of Patent Literature 1). Therefore, there arises a problem in that, as shown in FIG. 9(B) of Patent Literature 1, when an object S which linearly extends in a vertical direction moves in a horizontal direction while the object S is captured, the object S is tilted in a still image. Such an electronic shutter function of the CMOS image sensors is referred to as a “rolling shutter” or a “focal plain shutter”.
On the other hand, CMOS image sensors in which a shutter is simultaneously released for all rows so that an identical exposure time is obtained have been proposed (refer to FIG. 11 of Patent Literature 1, for example). In such a CMOS image sensor, photodiodes in all the rows are simultaneously reset at a certain time, and after a predetermined exposure time has elapsed, charges of the photodiodes of all the rows are simultaneously transmitted to floating diffusions (FDs). Then, signals transmitted to the FDs are output on a row-by-row basis. In this manner, as shown in FIG. 11(B) of Patent Literature 1, even when the object S which linearly extends in the vertical direction moves in the horizontal direction, the object is not tilted in the captured still image.
Citation List
Patent Literature
PTL 1 Japanese Patent Laid-Open No. 2006-191236
PTL 2 Japanese Patent Laid-Open No. 2005-176105
However, in pixels (FIG. 10 of Patent Literature 1) of the CMOS image sensor which realizes FIG. 11 of Patent Literature 1, it is difficult to totally block light to be incident on components other than light incident on the photodiodes. That is, by the time the signals of a row of FDs are output, light is leaked to adjacent rows of FDs, and therefore, an amount of light leakage in a preceding row from which signals are output is different from an amount of light leakage in a succeeding row from which signals are output. Therefore, there arises a problem in that a difference among the amounts of light leakage is generated and degradation of a captured image occurs.
In order to address the difference among the amounts of light leakage, an image pickup apparatus which controls resetting of photodiodes and transmission to FDs so that the resetting of the photodiodes and the transmission to the FDs are in synchronization with a light-shielding operation of a mechanical shutter by making a reading direction in a unit of row of a CMOS image sensor equal to a light-shielding direction of the mechanical shutter has been proposed (refer to Patent Literature 2, for example). As shown in FIG. 3 of Patent Literature 2, periods of time from when the signals are transmitted to the FDs to when the mechanical shutter performs light shielding for all the rows become the same as one another, and therefore, the problem of the difference among the amounts of light leakage appears to be solved.
However, as shown in FIG. 3 of Patent Literature 2, since the resetting of the photodiodes and the transmission of the signals to the FDs for all the rows are not simultaneously performed, a moving object is captured in a tilted state.
The present invention has been made in view of the above problems, and an object of the present invention is to provide an image pickup apparatus capable of preventing distortion of images captured by an image pickup element which employs an XY reading method and capable of suppressing degradation of captured images caused by leakage of light to pixels.